In vitro diagnostics (IVD) allows labs to assist in the diagnosis of disease based on assays performed on patient fluid samples. IVD includes various types of analytical tests and assays related to patient diagnosis and therapy that can be performed by analysis of a liquid sample taken from a patient's bodily fluids, or abscesses. These assays are typically conducted with automated clinical chemistry analyzers (analyzers) into which tubes or vials containing patient samples have been loaded. Because of the variety of assays needed in a modern IVD lab, and the volume of testing necessary to operate a lab, multiple analyzers are often employed in a single lab. Within analyzers, there may be multiple stations used for preparation and analysis tasks. Accordingly, fluid samples are often carried in sample vessels, such as tubes, that are placed into carriers or trays throughout the laboratory system, manually or as part of an automation system.
During the handling of samples, tubes must routinely be removed from and placed into slots in these carriers and trays. Due to the volume at which samples need to be processed in a typical IVD environment, the task of handling sample vessels, to insert and remove these tubes from tube slots in trays and carriers, is handled by sample handling robot arms. These sample handling arms are generally automated and must detect the location of a sample slot and need to place a sample into the slot without damaging the sample.
Currently in laboratory automation there are a couple of approaches for detecting whether a test tube or sample tube has been firmly placed where it should be (e.g., inside of a holder/tube slot). Most laboratory automation systems use some type of spring-holder. The spring(s) can center a tube in the slot or press the tube to a side in a cavity that defines the tube slot. These tube holders could be part of pucks/carriers, part of trays, or located elsewhere in the instrument.
Fundamentally, most of the sample-handling robot arm systems and pick and place devices are stepper-motor-based systems. A couple of techniques are used to ensure placement without breaking the sample tube. A sensing unit can include a spring sensor, allowing mechanical detection and some compliance. Generally these sensors are built into the robot arm, because the trays and carriers are generally passive devices. When the tube is pushed down, the tube causes a certain deflection in the spring, and it triggers a sensor that identifies that the tube has compressed the spring by some predetermined amount. This amount can indicate that the sample handler has successfully seated the tube, and the automated system can then take action. However, this is an indirect measurement. All the system can tell is that it pushed a sample tube against something that applied enough force to trigger the sensor. Accordingly, mechanical errors could also trigger such a sensor.
A second approach is to keep everything in the sample handling system at effectively the same height, allowing the tube to be moved around with respect to a plane. If you assume little or no manufacturing deviation, variations in height can be fairly minimal and inconsequential. In these systems, a handling device can be taught or programmed to move the tube in an open loop fashion with respect to the plane, monitoring the x-y-z coordinates of the handling device. Because these systems typically lack feedback from sensors in tube slots, their operation assumes that moving a tube to a given location results in proper placement. It can be difficult to determine if positions have drifted, which can affect performance. These systems are often servo motor based.
Servo-based systems can generally move much faster than stepper motor systems. However, servos typically encounter problems when relying on mechanical crash/crush sensors. Because servo-based systems tend to move much faster than stepper-based systems, sensors may only alert the system of an error after a tube has collided with a tube holder. Accordingly, servo-based systems tend to be limited to open loop, position-based placement systems.
Accordingly, there are two general options for confirming to placement in the prior art. One is a closed loop system where a contact sensor on the arm or in the tube receptacle confirms contact between a tube and a tube slot. This would be typically used with a pick-n-place device or a robot arm that is stepper motor driven. Typically the systems using a closed loop move much slower than servo-based systems, because the lag between detection and halting. The additional time needed for processing mechanical sensor feedback generally necessitates slower point-to-point motion.
The second option is to use conventional distance/position detection means, such as encoding, to determine the position of the end effectors on the robot arm. This type of system would be termed an open loop system because there is no sensory feedback. Such systems can be useful for stepper systems or servo based robot arms or pick and place devices.
Existing systems may have certain drawbacks. Sensor-based closed-loop systems often have extra mass for the sensor unit on the arm. If you have these extra sensors, systems need bigger robots and more power in that to move them around to do their function. Systems end up adding more mass and over designing to compensate for sensors on the robot arm. This can also create larger power supply demand. There may more cabling, requiring bigger e-chains (a flexible chain that keeps the cables constrained in a moving device). Bigger e-chains means more resistance, more resistance means a higher power device which requires a larger power supply, etc.
Furthermore, stepper-based closed-loop systems tend to be fairly expensive. The individual sensing packages carry an increased cost, which includes the sensors and the auxiliary hardware and increased cost to build more robust mechanisms to support the sensors.
One drawback with conventional open-loop systems is that the system does not actually know where the tube is. If a device slips and the system doesn't detect it, it may mis-approximate where it is actually moving tubes. In an extreme example a system could completely miss an intended position by a wide margin, perhaps causing a sample tube to fall over, spill, or break. Such an error can be catastrophic in an IVD system as it can spread contamination to other systems and samples. Another error may occur where a tube is mis-positioned by a small amount, such that the tube sticks up higher than expected when it leaves the sample handling station. The tube top can then later encounter another mechanism in the automation system, crashing and breaking the test tube which can cause a sample splash or spill. This can lead to cross-contamination of a large area depending on the size of the device and the speed by which samples move. As long as open loop systems work as expected, they tend to work well, but can have catastrophic problems due to slight changes.